FIG. 18 schematically shows a cross sectional structure for explaining an example of an arrangement of a previously proposed GMR (Giant Magneto-Resistance) element making use of a GMR effect. For example, on a silicon substrate 200, a first electrode 201, a first ferromagnetic layer 203 (with a thickness of approximately 40 nm and a diameter of approximately 100 nm) made of a material such as Co, a nonmagnetic metal layer 204 (with a thickness of approximately 6 nm and a diameter of approximately 100 nm), a second ferromagnetic layer 205 (with a thickness of approximately 2.5 nm and a diameter of approximately 100 nm) made of a material such as Co, and a second electrode 206 are formed in this order. Further, a bit line 207 is formed on the first electrode 201. It is known that such a GMR element can reverse the direction of magnetization of the second ferromagnetic layer 205 by spin current injection from the second electrode 206 side, that is, injection of electrons with polarized spins from the first electrode 201 side. See for example JP-A-2004-207707 and J. A. Katine, et al., Current-Driven Magnetization Reversal and Spin-Wave Excitations in Co/Cu/Co Pillars, Physical Review Letters, Vol. 84, No. 14, pp. 3149-3152 (2000).
The operation principle of the element is explained as follows. First, a magnetic field with a sufficient strength is applied to the element to align the directions of magnetization of the first ferromagnetic layer 203 and the second ferromagnetic layer 205 in the same direction. FIG. 19A schematically show a cross sectional view of the element shown in FIG. 18 in which the direction of magnetization in the ferromagnetic layers are aligned rightwardly (arrows in the figure showing the direction of magnetization) in each ferromagnetic layer. In the following drawings, arrows have the same meaning. The state is to be referred to as the parallel state (P-state). In this state, an electric current flowing in the direction from the second electrode 206 side to the first electrode 201 side causes electrons to be injected from the first electrode 201 to the first ferromagnetic layer 203. In the first electrode 201, the electron spins are in a state in which the distribution of up-spins corresponds to that of down-spins. In the ferromagnetic layers, however, due to interaction (s-d interaction) between the electron spins and the spins of ferromagnetic metal atoms, the directions of the electron spins are aligned with the direction of magnetization of the first ferromagnetic layer. This is referred to as polarization of spin. Injection of electrons with thus polarized spins into the second ferromagnetic layer 205 through the nonmagnetic metal layer 204 exerts a torque on the magnetization of the ferromagnetic layer 205 in the direction expressed by following Equation (1):j·M(ferromagnetic layer 205)×M(ferromagnetic layer 203)×M(ferromagnetic layer 205)  (1),where j is a current (a scalar quantity), M(ferromagnetic layer 205) is a unit vector in the direction of the magnetization of the ferromagnetic layer 205, and M(ferromagnetic layer 203) is a unit vector in the direction of the magnetization of the ferromagnetic layer 203.
The torque expressed by Equation (1) is also exerted to the magnetization of ferromagnetic layer 203. The ferromagnetic layer 203, however, has a thickness sufficiently larger than the thickness of the ferromagnetic layer 205, so that the magnetization of the ferromagnetic layer 203 is unaffected. Therefore, a current exceeding a certain level of a critical current causes only the direction of the magnetization of the ferromagnetic layer 205 to rotate by the exerted torque, by which the state of the magnetization between the ferromagnetic layer 205 and the ferromagnetic layer 203 changes from the P-state shown in FIG. 19A to an anti-parallel state (AP state) shown in FIG. 19B.
An explanation is made when a current flows from the first electrode 201 to the second electrode 206 in the element in the AP-state. In this case, the sign of the current in Equation (1) expressing the direction of torque becomes negative, so that a torque in the direction opposite to the above is exerted on the magnetization of the ferromagnetic layer 205. As a result, a current exceeding a certain level of a critical current causes the direction of the magnetization of the ferromagnetic layer 205 to be inverted, by which the state of the magnetization in the element returns from the AP-state to the P-state shown in FIG. 19A. The electric resistance of a GMR element is known to be small in the p-state and large in the AP-state with the rate of change being several tens of percent. By using the GMR effect, a reading head can be manufactured for a hard disk. FIG. 20 is a schematic view showing a planar structure of an MRAM (Magnetic Random Access Memory) in which a plurality of the GMR elements shown in FIG. 18 are connected to use the inversion of magnetization of GMR elements by current injection. With the use of the arrangement as shown in FIG. 20, writing (inversion of magnetization) and reading (detection of electric resistance values corresponding to states of magnetization of recording cells 209) of bit information to and from the recording cells 209 are principally possible by a group of laterally running word lines 208 and a group of longitudinally running bit lines 207.
FIGS. 21, 22A, and 22B schematically illustrate cross sectional views each for explaining a phenomenon of displacement of a magnetic domain wall formed in a ferromagnetic wire in a related magnetic domain wall displacement element by a current flowing in the ferromagnetic wire. See for example A. Yamaguchi, et al., Real-Space Observation of Current-Driven Domain Wall Motion in Submicron Magnetic Wires, Physical Review Letters, Vol. 92 No. 7, 077205 (2004). FIG. 21 is a schematic cross sectional view showing an arrangement of the element, in which a ferromagnetic layer 221 (10 nm in thickness and several micrometers in length) is formed on an insulator substrate 220. On the ferromagnetic layer 221, a first electrode 222 and a second electrode 223 are formed. For the ferromagnetic layer 221, a material such as a permalloy (Ni81Fe19) thin film is used. For the first and second electrodes 222 and 223, a material such as copper (Cu), gold (Au), or platinum (Pt) is used. FIGS. 22A and 22B are schematic cross sectional views for explaining the principle of displacement of a magnetic domain wall 224 when a current flows between the first electrode 222 and the second electrode 223. In each of the views, the directions of magnetization in the magnetic layer are shown with arrows like in the above explanation.
First, as shown in FIG. 22A, consider the case in which there is one magnetic domain wall 224 in the region of the ferromagnetic layer 221 between two electrodes and the direction of magnetization on the right side of the magnetic domain wall 224 is opposite to the direction of magnetization on the left side. When flowing a current in this state from the second electrode 223 to the first electrode 222, the current crosses the magnetic domain wall 224. At that time, electrons are injected from the first electrode 222 into the ferromagnetic layer 221 to flow into the second electrode 223. The directions of spins of electrons injected into the ferromagnetic layer 221 are considered to be aligned by s-d interaction in the same direction as the direction of magnetization in the region on the left side of the magnetic domain wall 224 in the ferromagnetic layer 221 (also referred to as polarization). The magnetization due to spins of the polarized electrons is taken as Sl (a rightward vector). Then, when the spin-polarized electrons pass through the magnetic domain wall 224 and are injected into the region on the right-hand side of the magnetic domain wall 224 of the ferromagnetic layer 221, the directions of spins of electrons is to be aligned this time by s-d interaction in the same direction as the direction of magnetization opposite to the direction before the electrons pass through the magnetic domain wall 224. The magnetization due to spins of the electrons polarized on the right-hand side of the magnetic domain wall 224 is taken as Sr (a leftward vector). Moreover, the magnetization on the left-hand side of the ferromagnetic layer 224 and the magnetization on the right-hand side are taken as Ml (a rightward vector) and Mr (a leftward vector), respectively.
As was explained above, with the direction of Sr considered to be positive, in the process in which electrons move from the left-hand side to the right-hand side of the magnetic domain wall 224, magnetization Sr due to electron spin changes to Sl to result in an increase in electron spins in the negative direction. Before and after electrons cross the magnetic domain wall, the total sum (Ml+Sl+Mr+Sr) of spin angular momentum of magnetization of the magnetic material and conduction electrons is conserved to be constant. In a process in which conduction electrons on the left-hand side of the magnetic domain wall cross the magnetic domain wall, the total sum of whole spin angular momentum of electrons (Sl+Sr) increases by 2Sr (decreases by 2Sl). That is, by the conduction electrons crossing the magnetic domain wall 224 from the left-hand side to the right-hand side, the magnetization M1 of the magnetic domain wall is to go on increasing (the magnetic domain wall 224 is to go on moving in the same direction as the direction in which electrons flow).
FIGS. 22A and 22B show the difference in position of the magnetic domain wall 224 between the state before a current is made to flow from the electrode 223 and the state after a current is made to flow from the electrode 223. In this way, it is known that the magnetic domain wall 224 moves in the direction opposite to the direction in which the current flows. It is reported that the current density enabling the displacement of the magnetic domain wall is of the order of 108 A/cm2 in the case of metallic magnetic material such as permalloy and of the order of 8×104 A/cm2 in the case of ferromagnetic semiconductor and that, by increasing a current density, the displacement speed of the magnetic domain wall becomes of the order of 3 m/s. See for example Yamaguchi's paper and Michihiko Yamanouchi, Abstract for 60th Annual Meeting Phys. Soc. Jpn., p. 27aYP-5, Mar. 27 (2005).
Each of the above-explained two technologies inverts the magnetization direction by flowing a current in the element. Its operation principle is based on the fact that, when spin-polarized electrons are injected into a ferromagnet, a torque due to electron spin is exerted on the magnetization of the ferromagnet. At this time, the total of the magnetization due to spins of the injected free electrons and the magnetization of the ferromagnet is conserved. Thus, for bringing about inversion of magnetization with a slight amount of injected electrons (or an injected current), the volume and the magnitude of saturation magnetization of the ferromagnet subjected to inversion of magnetization must be made small.
For example, in the case of the MRAM shown in FIG. 20, when its volume and its saturation magnetization are made small, a problem arises in that thermal stability of recording bit, namely thermal stability of magnetization of the recording cell 209, becomes low, causing thermal fluctuation of magnetization by thermal disturbance, even at room temperature, and making it impossible to keep the magnetization of the recording cell. Also in the arrangement shown in FIG. 21, for carrying out high speed displacement of the magnetic domain wall by a slight current, saturation magnetization must be lowered. However, lowering the magnetization saturation increases thermal fluctuation of magnetization forming the magnetic domain wall. Thus, it can be easily supposed that a problem arises in which the position of the magnetic domain wall is randomly displaced by thermal agitation.
Furthermore, with the structure shown in FIG. 21, although it is possible to induce a change in a state of magnetization, i.e., displacement of the magnetic domain wall, by supplying a current, it is difficult to detect a state of magnetization. This is because, in the case of the arrangement shown in FIG. 20, only the position of the magnetic domain wall changes without change in the length of the magnetic layer in which a current flows. Although the ratio of the length of the region magnetized rightward and the length of the region magnetized leftward changes in the ferromagnet 221, it is considered that the rightward resistivity and the leftward resistivity are the same. Therefore, the difference in the electric resistance due to change in the ratio of the lengths is in a negligible level. Hence, only with such displacement of the magnetic domain wall, there is no large change in the electric resistance between both of the electrodes.
Accordingly, there remains a need for an element in which the magnetized state of the element can be changed by flowing a current between two electrodes and changing the electric resistance between the two electrodes depending on the magnetized state of the element, to provide the element as one in which thermal stability of the magnetized state of the element is improved, while a critical current necessary for changing the magnetized state remains small. Also, there remains a need for an element in which a magnetic domain wall is displaced by flowing a current between two electrodes of a magnetic material, to provide the element as one in which the electric resistance between the two electrodes is changed by displacement of the magnetic domain wall. The present invention addresses these needs.